Dynamic testing in columns for soil heavy metal removal for a car park SUDS

The increase in urban runoff brought about by a rise in impermeable surfaces has triggered the alteration and pollution of many aquatic systems. The overall goal of this research was to design a 'Sustainable Urban Drainage System' (SUDS) for the retention of heavy metals from a car park consisting of mixing autochthonous soil (70%) with sand (30%) to improve the hydrological conductivity and adsorption capacity. To quantify the retention of metals we characterize the adsorption kinetics and isotherms of the soil mixture and perform dynamic experiments. The proposed methodology allowed us to work out the amount of heavy metal retention by the adsorbent and the retention mechanisms. The retention capacity of the adsorbent mixture was as follows: Cr³⁺ ≈ Cu²⁺ ≫ Zn²⁺ > Ni²⁺ > Cd²⁺. Chromium and copper ions were mainly retained by precipitation, whereas zinc, nickel and cadmium were retained by ionic exchange with calcium ions that saturate the soil colloids. The soil mixture buffered pH was found to change when fed with an acid solution of metallic ions.